A technique for forming a transistor by using a semiconductor layer formed over a substrate having an insulating surface has attracted attention. The transistor has been widely used for semiconductor devices such as integrated circuits and display devices. A silicon-based semiconductor layer is known as a semiconductor layer applicable to a transistor.
Whether an amorphous silicon layer or a polycrystalline silicon layer is used as a semiconductor layer in a transistor depends on the purpose. For example, in the case of a transistor included in a large display device, an amorphous silicon layer, which can be formed using an established technique for forming a film over a large substrate, is preferably used. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, a polycrystalline silicon layer, which can form a transistor having a high field-effect mobility; is preferably used. As a method for forming a polycrystalline silicon layer, high-temperature heat treatment or a laser light treatment which is performed on an amorphous silicon layer has been known.
Further, in recent years, an oxide semiconductor layer has attracted attention. For example, a transistor which includes an oxide semiconductor layer containing indium, gallium, and zinc and having a carrier density lower than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor layer can be formed by a sputtering method, and thus can be used for a transistor in a large display device. A transistor including an oxide semiconductor layer has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. Moreover, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
A transistor including an oxide semiconductor layer is known to have extremely low leakage current (also called off-state current) in an off state. For example, a low-power-consumption CPU utilizing such a low leakage current characteristic of a transistor including an oxide semiconductor layer is disclosed (see Patent Document 2).